1. Field of the Invention
The present invention relates to a planispherical parallax-free X-ray imager more particularly adapted to industrial and/or medical application.
2. Brief Description of the Invention
Planispherical X-ray imaging devices have been up to now investigated. Most important work concerning that particular subject matter was developed by Georges CHARPAK at the EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH in Geneva (Switzerland).
A first development concerned the properties of proportional chambers with spherical drift spaces.
A proportional wire chamber equipped with a resistive divider adapted to generate appropriate spherical equipotential surfaces within the drift space of the wire chamber has been first disclosed by G. CHARPAK, Z. HAJDUK, A. JEAVONS, R. STUBBS--CERN, Geneva, Switzerland, and R. KAHN, Centre Multidisciplinaire Paris XII, av. General de Gaulle, Creteil, France, and edited by NUCLEAR INSTRUMENTS AND METHODS 307 (1974)--Geneva, Jul. 29, 1974.
A proportional wire chamber embodied as a large aperture X-ray imaging chamber equipped with a spherical drift space has been also disclosed by G. CHARPAK, C. DEMIERRE, R. KAHN, J-C. STANDIARD and F. SAULI at the CERN in Geneva. See NUCLEAR INSTRUMENTS AND METHODS 141 (1977) 449-455, North-Holland Publishing Co. A spherical drift space is disclosed as to embodying entrance and exit electrodes of spherical shape with an angular acceptance for X-rays to 90.degree.. Coupling of spherical drift space and readout proportional chamber is disclosed to consist of a transfer space T, the lateral wall of which comprises a resistive divider adapted to generate spherical equipotential surfaces of increasing radius up to the first cathode electrode of the readout proportional chamber.
A general survey on various methods of correction for parallax errors on gaseous detectors for X-rays and UV has been published by G. CHARPAK, CERN, Geneva, Switzerland. See NUCLEAR INSTRUMENTS AND METHODS 201 (1982) 181-192, North Holland Publishing Company.
More recently, P. REHAK, G. C. SMITH and B. YU, Brookhaven National Laboratory, Uptown N.Y. 11973 presented a method for reduction of parallax broadening in gas-based position sensitive detectors at the 1996 IEEE Nuclear Science Symposium, Anaheim, Calif., Nov. 2-9, 1996 and published as IEEE Transactions on Nuclear Science, vol.44, No. 3, 1997, 651-655.
Although the drift space for photons is confined within an entrance electrode and the cathode wire plane of the readout chamber are plane and parallel, entrance window of the readout chamber is further provided with a particular conductive pattern adapted to introduce progressive bending of the equipotential surfaces, electric field lines crossing thus this equipotential surfaces at right angle, whichever the impinging direction of X-rays emanating from the focal point, so as to correct and reduce any parallax error.
In a general point of view, the above mentioned X-ray imagers may prove satisfactory to the extent that the parallax error is now reduced to a few percent. Embodying the entrance window of the readout chamber with conductive pattern adapted to provide full correction of parallax error is quite difficult to implement, since actual pattern and corresponding voltage which is to be applied to these conductive patterns are such that the electric field is approximately radial only close to the ring patterned entrance window, while it becomes substantially parallel in approaching the equipotential second electrode which defines the conversion volume. As a consequence, parallax error is thus increasing with penetration of the converting X-rays.